Setup shots add a separate startup quantity to a resin estimate. Part rejection increases the production quantity needed to deliver the required number of acceptable parts. Calculate production demand using yield, then add startup consumption—not the other way around.
Keep these allowances separate. Startup consumption depends on how many starts the job requires and how much material each start consumes. Steady-state rejection applies to the parts molded after the process is released for production. Combining both into one scrap percentage makes the estimate harder to check and can count the same losses twice.
Establish what each shot and rejection rate includes
For a mold producing identical parts, collect these inputs:
| Input | Definition | Unit |
|---|---|---|
| Required good parts, (Q) | Accepted parts needed to fulfill the order | parts |
| Active cavities, (C) | Cavities producing the ordered part each production cycle | parts/cycle |
| Part weight, (m_p) | Resin weight of one molded part, excluding separately counted runner material | g/part |
| Runner weight, (m_r) | Total sprue and cold-runner material ejected each cycle | g/cycle |
| Production rejection rate, (r) | Rejected production parts divided by all production parts molded, excluding separately counted setup output | decimal |
| Setup shots, (S) | Additional molding cycles whose output does not count toward the good-part order | cycles |
Use resin weights, not the combined weight of resin and metal inserts. Use the active cavity count rather than the mold’s nominal cavity count if cavities are blocked.
The full-shot resin weight is:
[
m_s=Cm_p+m_r
]
Here, (m_s) is in grams per cycle. If the measured shot weight already includes all parts, the sprue, and runners, use that value directly; do not add runner weight again.
For a hot-runner mold, include any cold sprue or runner actually ejected with each shot. Do not charge the entire hot-runner manifold inventory to every cycle. Account separately for material consumed in filling, clearing, or emptying the system when those operations require additional resin.
This calculation assumes a stable cavity configuration and approximately constant full-shot weight during production. A family mold producing different part numbers requires a separate output balance for each part number; one combined good-part count may not satisfy the required product mix.
Calculate production shots from acceptable-part yield
When (r) is the fraction of production parts rejected, the expected acceptable output per cycle is:
[
C(1-r)
]
The planned number of production cycles is therefore:
[
N_{\text{production}}
=
\left\lceil
\frac{Q}{C(1-r)}
\right\rceil
]
The ceiling symbol means round up to the next whole cycle. Enter 3% rejection as (0.03), not (3).
Production resin consumption is:
[
M_{\text{production}}
=
\frac{N_{\text{production}}m_s}{1000}
]
With shot weight in grams, the result is kilograms.
Dividing by yield is not the same as adding the rejection percentage to the good-part quantity. At 3% rejection, the production multiplier before whole-cycle rounding is:
[
\frac{1}{1-0.03}=1.03093
]
Multiplying by (1.03) slightly understates the requirement because replacement parts are also exposed to rejection.
Check the denominator in historical reports. “Rejected parts divided by total molded parts” is different from “scrap weight divided by material issued.” A weight-based scrap figure may already include runners, startup material, and purge. It cannot be inserted into this part-rejection formula without reconciling those categories.
The calculated cycle count is a planning estimate based on average yield, not a guarantee of sufficient accepted output. Actual production should be reconciled against accepted-part counts.
Add setup consumption outside the rejection calculation
If each setup shot consumes approximately one full production shot:
[
M_{\text{setup}}=\frac{Sm_s}{1000}
]
Then:
[
M_{\text{molding}}
=
M_{\text{production}}+M_{\text{setup}}
]
Do not divide setup consumption by production yield. These setup shots are already additional cycles whose output has been excluded from the delivery quantity.
Full-shot weight is only an allowance for startup. Fill-only trials, short shots, and changing settings may consume different amounts. When recorded startup weights are available, use the measured total instead:
[
M_{\text{setup}}
=
\frac{\sum m_{\text{setup},i}}{1000}
]
Here, (m_{\text{setup},i}) is the resin consumed by each setup shot in grams.
Set a clear boundary between setup and production. If acceptable startup parts are released and counted toward the order, subtract those parts from the remaining good-part requirement and account for the material used to produce them once. Do not count them as both fully rejected setup output and delivered production.
Count starts, not just jobs. A run split across multiple machine starts may need a separate allowance for each start. Distinguish an initial material changeover from a restart with the same resin already in the machine; use records for the relevant condition rather than assuming equal consumption.
Worked example: 10,000 good parts with 3% production rejection
The following values are hypothetical planning inputs, not recommended process allowances:
| Input | Assumed value |
|---|---|
| Required good parts | 10,000 |
| Active cavities | 4 |
| Resin weight per part | 12 g |
| Total sprue and cold-runner weight | 8 g/cycle |
| Steady-state part rejection | 3% |
| Setup shots | 40 |
| Average setup-shot weight | 56 g |
| Regrind returned to this run | None |
The full-shot weight is:
[
m_s=(4\times12)+8=56\text{ g/cycle}
]
The production cycle requirement is:
[
N_{\text{production}}
=
\left\lceil
\frac{10{,}000}{4(1-0.03)}
\right\rceil
=
2{,}578\text{ cycles}
]
Production resin is:
[
M_{\text{production}}
=
\frac{2{,}578\times56}{1000}
\approx144.37\text{ kg}
]
Setup consumption is:
[
M_{\text{setup}}
=
\frac{40\times56}{1000}
=
2.24\text{ kg}
]
Total molding consumption is approximately:
[
M_{\text{molding}}=144.37+2.24=146.61\text{ kg}
]
The calculation assumes rejected parts still consume the normal full-shot resin allowance. If actual reject cycles consistently consume less or more material, use measured consumption to refine that assumption.
Identify what increased the estimate
With no setup allowance and no rejection, the same order would require 2,500 cycles and 140 kg of resin, including the runners.
| Component | Resin quantity |
|---|---|
| Baseline production, including runners | 140.00 kg |
| Additional production caused by rejection, including cycle rounding | 4.37 kg |
| Setup shots | 2.24 kg |
| Total molding consumption | 146.61 kg |
This breakdown shows where the difference comes from. A blanket percentage would obscure whether the additional resin is associated with startup or ongoing quality loss.
Setup consumption is a fixed addition for a given number of starts. Its percentage impact becomes larger on short orders. Rejection, by contrast, increases consumption across the production run.
Convert molding consumption into a purchase quantity
Molding consumption is not necessarily the amount to order. Add other consumption of the same resin that is not already included, then reconcile usable inventory, purchasing reserve, and package size.
A simple no-regrind purchasing balance is:
[
M_{\text{net}}
=
\max\left(
0,,
M_{\text{molding}}
+M_{\text{purge}}
+M_{\text{other}}
+B-I
\right)
]
All terms are in kilograms:
- (M_{\text{purge}}): additional production resin consumed during purging or changeover.
- (M_{\text{other}}): separately estimated consumption or unrecoverable loss not counted elsewhere.
- (B): an explicit purchasing reserve.
- (I): usable, uncommitted inventory of the required resin available before production.
Keep commercial purge compound separate from the production-resin order. Also check whether startup records already include production resin purged through the nozzle; adding it again would duplicate the allowance.
Purge requirements depend on the material and interruption conditions. For example, BASF’s Elastollan injection-molding guidance calls for purging material from the cylinder before resuming after longer interruptions because retained material can overheat. That guidance concerns Elastollan processing; it does not establish a universal purge quantity for other resins.
Continuing the example, assume:
- 3 kg of additional production resin for purging.
- 1 kg of separately estimated handling loss.
- A 5 kg purchasing reserve.
- 20 kg of usable, uncommitted inventory.
- Supply in whole 25 kg bags.
Estimated consumption before reserve and inventory is:
[
146.61+3+1=150.61\text{ kg}
]
The net purchase requirement is:
[
150.61+5-20=135.61\text{ kg}
]
Rounding up to whole 25 kg bags gives a purchase order of 150 kg.
Keep the 5 kg reserve visible. It is a purchasing decision, not another predicted process loss. Likewise, package rounding increases the purchase quantity without increasing expected consumption.
Do not subtract all generated scrap as a regrind credit
The worked example assumes no material is returned to the run. If runners or rejected parts are reused, distinguish total material passing through the machine from new resin required.
Do not simply subtract every kilogram of generated scrap. Before assigning a credit, establish:
- Whether reuse is permitted for the specified material and part.
- How much recovered material is actually usable.
- The approved blend proportion.
- When that material will be available.
- Whether it has already been counted in available inventory.
Scrap generated late in the run cannot supply earlier production. A regrind plan therefore needs a time-based virgin/regrind balance, not just a final scrap-weight deduction. If those inputs are unavailable, keep the purchase estimate on a no-reuse basis and show potential recovery separately.
Replace allowances with job records on repeat orders
Record startup consumption separately from production rejection, then reconcile the estimate against material issued, returned inventory, accepted output, and scrap.
For a repeat job, the most useful records are:
- Number and type of starts, including changeovers and restarts.
- Setup-shot count and actual startup resin weight.
- Production resin used for purge, recorded separately from purge compound.
- Active cavity count and measured full-shot weight.
- Total production parts, rejected parts, and accepted parts.
- Material returned to stock and regrind actually reused.
Calculate the steady-state rejection rate from production counts only:
[
r=
\frac{\text{rejected production parts}}
{\text{total production parts molded}}
]
If startup output is already recorded separately, exclude it from both counts. Use records from comparable resin grades, colors, molds, and operating conditions, and retain separate allowances where those conditions materially change consumption.
This keeps the next resin order traceable: production yield determines the replacement output, startup records determine the setup allowance, and the purchasing balance explains the difference between estimated consumption and the quantity ordered.